The present invention relates to a ferroelectric capacitor device including a ferroelectric film having a bismuth layer structure as a capacitor insulating film.
In recent years, with the advance of digital technology, the tendency to process and store a massive amount of data has been accelerated. In this situation, electronic equipment has been further sophisticated, and rapid progress has been made in increasing the integration of semiconductor integrated circuit devices used for electronic equipment and attaining finer semiconductor elements.
To realize higher integration of dynamic random access memories (RAMs), there has been widely researched and developed a technology of using a dielectric having a high dielectric constant (hereinafter, simply called a high dielectric) as a capacitor film of a memory capacitor device, in place of a silicon oxide or a silicon nitride conventionally used.
Research and development have also been vigorous on ferroelectric films having the spontaneous polarization property, with the aim of commercializing a nonvolatile RAM that can operate at a lower voltage than is conventionally allowed and permit write/read at a high speed.
As a ferroelectric film used for a nonvolatile RAM, promising is a ferroelectric film having a bismuth layer structure, which is excellent in rewrite endurance and can operate at a low voltage. In general, a bismuth layer structure is represented by chemical formula (a):
(Bi2O2)(Amxe2x88x921BmO3m+1)xe2x80x83xe2x80x83(a)
where m is an integer equal to or more than 1, A is a univalent, divalent or trivalent metal, and B is a tetravalent, pentavalent or hexavalent metal.
The above bismuth layer structure includes bismuth oxide layers (Bi2O2) and perovskite-like layers (Amxe2x88x921BmO3m+1) alternately put on top of each other.
Among a group of materials having the bismuth layer structure, a material called SBT, in particular, is often used for nonvolatile memories.
The SBT is a compound represented by chemical formula (b):
(Bi2O2)(SrTa2O7)xe2x80x83xe2x80x83(b),
that is, m is 2, A is divalent Sr, and B is pentavalent Ta in chemical formula (a) above (hereinafter, this compound is called a normal type).
The laminated structure of the compound is as shown in FIG. 15, in which bismuth oxide layers 101 and perovskite-like layers 102 (m=2) are alternately put on top of each other.
The bismuth oxide layer 101 (chemical formula: Bi2O2) has a structure as shown in FIG. 16, in which square pyramids linked to one another extend two-dimensionally. Bismuth 111 exists at the apex of each square pyramid, and oxygen 112 exists at each corner of the bottom square of the square pyramid.
The m=2 perovskite-like layer 102 (chemical formula: SrTa2O7) has a layer structure as shown in FIG. 17, in which oxygen octahedra extend two-dimensionally with each two placed one upon the other vertically. Tantalum 113 exists in the B site as the center of each oxygen octahedron, and oxygen 112 exists at each apex of the oxygen octahedron. Strontium 114 exists in the A site as a space surrounded by the oxygen octahedra.
The SBT has problems to be tackled. The first problem is improving the spontaneous polarization amount, and the second problem is suppressing the leakage current and improving the breakdown voltage. As methods for improving the spontaneous polarization as the first problem, the following two crystal structures (a mixed layered superlattice type and an A-site Bi substitution type) have been proposed.
(1) Mixed layered superlattice type layer structure (first prior art)
The layer structure of this type is disclosed in U.S. Pat. No. 5,955,754 to Azuma et al. This literature describes extensively the entire of the layer structure. Herein, however, the disclosed layer structure will be described as being applied to SBT according to the purport of the present invention. As shown in FIG. 18, the mixed layered superlattice type layer structure (this is not a commonly-accepted name but is called herein for convenience to distinguish from other structures) includes either a perovskite-like layer 102 (m=2) or a perovskite-like layer 103 (m=1) interposed between every two adjacent bismuth oxide layers 101. When the existence probability of the m=1 perovskite-like layer 103 is xcex4 (0 less than xcex4 less than 1), the existence probability of the m=2 perovskite-like layer 102 is 1xe2x88x92xcex4.
The m=1 perovskite layer 103, represented by TaO4, has a layer structure as shown in FIG. 19, in which a single layer of oxygen octahedra having tantalum 113 as the center extends two-dimensionally. The tantalum 113 exists in the B site as the center of each oxygen octahedron, and oxygen 112 exists at each apex of the oxygen octahedron. If valence calculation is made strictly, the chemical formula should be TaO7/2, indicating that the oxygen amount is short to form the structure shown in FIG. 19. A vacancy is therefore formed in an oxygen-lacking portion.
The feature of the mixed layered superlattice is that because the amount of bismuth having a low melting point is large compared with the normal structure, crystal grains can be easily grown large, and this can improve the spontaneous polarization property.
(2) A-site Bi substitution type layer structure (second prior art)
The layer structure of this type, disclosed in Japanese Laid-Open Patent Publication No. 9-213905 to Atsugi et al., is represented by chemical formula (c):
(Bi2O2)[(Sr1xe2x88x92xBix)Ta2O7]xe2x80x83xe2x80x83(c)
The A-site Bi substitution type layer structure includes the bismuth oxide layers 101 and the m=2 perovskite-like layers 102 alternately put on top of each other as shown in FIG. 15.
The bismuth oxide layer 101, represented by chemical formula: Bi2O2, has the structure shown in FIG. 16 as in the normal type.
The m=2 perovskite-like layer 102, represented by chemical formula: (Sr1xe2x88x92xBix)Ta2O7, has a structure shown in FIG. 20. The structure shown in FIG. 20 resembles the structure shown in FIG. 17, in which tantalum 113 exists in the B site as the center of each oxygen octahedron, and oxygen 112 exists at each apex of the oxygen octahedron. The difference is that the A site 115 is occupied by Sr with a probability of (1xe2x88x92x) or Bi with a probability of x. That is, while all the A sites 115 are occupied by Sr in the normal type, Bi substitutes for Sr in the A sites 115 with a probability of x.
In a recent research, formation of a vacancy in the A site 115 has been confirmed. The reason is that since trivalent Bi substitutes for divalent Sr, a vacancy is formed to satisfy the charge neutrality law. In this case, chemical formula (c) is changed to chemical formula (d):
(Bi2O2)[(Sr1xe2x88x92xBi2x/3)Ta2O7]xe2x80x83xe2x80x83(d)
Thus, in the A-site Bi substitution type, the m=2 perovskite-like layer 102 is represented by chemical formula: (Sr1xe2x88x92xBi2x/3)Ta2O7, where the A site shown in FIG. 20 is occupied by Sr with a probability of (1xe2x88x92x), Bi with a probability of (2x/3), or a vacancy with a probability of (x/3).
The feature of the A-site Bi substitution type is that since Bi3+ small in ion radius substitutes for Sr2+ in the A site 115, the lattice distortion increases, and this increases the spontaneous polarization amount. In addition, as in the mixed layered superlattice type, since the amount of Bi having a low melting point is large compared with the normal type, crystal grains can be easily grown large, and this can improve the spontaneous polarization property.
As described above, the first and second prior art structures can solve the first problem of SBT of improving the spontaneous polarization.
However, the first and second prior art structures fail to solve the second problem of SBT of reducing the leakage current and improving the breakdown voltage, for the following reason.
The first and second prior art structures cause generation of a precipitation at grain boundaries and the electrode interfaces. Specifically, a precipitation of Bi is generated in the mixed layered superlattice type layer structure of the first prior art, and a precipitation of BiTaO4 is generated in the A-site Bi substitution type layer structure of the second prior art. The precipitation acts as a leak path at grain boundaries resulting in increase of the leakage current, and lowers a Schottky barrier at the electrode interfaces causing decrease of the breakdown voltage.
As described above, the first and second prior art structures have the problem that they fail to obtain a capacitor device having the degree of reliability required for commercialization because the ferroelectric films used are prone to cause increase in leakage current and decrease in breakdown voltage.
An object of the present invention is providing a ferroelectric capacitor device including a ferroelectric film having a bismuth layer structure as a capacitor insulating film, capable of preventing occurrence of a failure due to increase in leakage current and decrease in breakdown voltage.
To attain the object described above, the present invention adopts a structure combining the mixed layered superlattice type and the A-site Bi substitution type to realize a ferroelectric film having a bismuth layer structure free from generation of a precipitation.
The first ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers includes two or more kinds of layers represented by general formula (1): Amxe2x88x921BmO3m+xcex1 (where A is a univalent, divalent or trivalent metal, B is a tetravalent; pentavalent or hexavalent metal in is an integer equal to or more than 1, at least one of A being Bi if in is an integer equal to or more than 2, and 0xe2x89xa6xcex1xe2x89xa61) and different in the value of m from each other.
According to the first ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
The second ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers include at least one first layer represented by general formula (2): BO3+xcex1 (where B is a tetravalent, pentavalent or hexavalent metal and 0xe2x89xa6xcex1xe2x89xa61) and at least one second layer represented by general formula (3): Amxe2x88x921BmO3m+1 (where A is a univalent, divalent or trivalent metal, and m is an integer equal to or more than 2, at least one of A being Bi).
According to the second ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film. Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
The third ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers include at least one first layer represented by general formula (4): BO7/2 (where B is a pentavalent metal) and at least one second layer represented by general formula (5): (A1xe2x88x92xBi2x/3)B2O7 (where A is a divalent metal, B is a pentavalent metal, and 0 less than x less than 1).
According to the third ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film. Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
Preferably, in the general formulae (4) and (5) of the third ferroelectric capacitor device, A is Sr, B is Ta1xe2x88x92yNby (where 0xe2x89xa6yxe2x89xa61).
By the above setting, a ferroelectric film excellent in fatigue characteristic can be used as the capacitor insulating film. This makes it possible to provide a ferroelectric capacitor device excellent in rewrite endurance.
In the third ferroelectric capacitor device, preferably, the proportion of the first layer in the plurality of perovskite-like layers is greater than 0 and smaller than 0.3, and 0 less than x less than 0.3 in the general formula (5).
By the above setting, generation of a precipitation can be suppressed substantially completely. This ensures the prevention of the ferroelectric capacitor device from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
The fourth ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers include at least one first layer represented by general formula (6): B1O7/2 (where B1 is a pentavalent metal) and at least one second layer represented by general formula (7): (A1xe2x88x92xBix)B12xe2x88x92xB2xO7 (where A is a divalent metal, B1 is a pentavalent metal, B2 is a tetravalent metal, and 0 less than x less than 1).
According to the fourth ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film. Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage. In addition, since formation of a vacancy in the A site can be suppressed, degradation of reliability such as rewrite endurance can be prevented.
Preferably, in the general formulae (6) and (7) of the fourth ferroelectric capacitor device, A is Sr, B1 is Ta1xe2x88x92yNby (where 0xe2x89xa6yxe2x89xa61), and B2 is Ti.
By the above setting, a ferroelectric film excellent in fatigue characteristic can be obtained. This makes it possible to provide a ferroelectric capacitor device excellent in rewrite endurance.
In the fourth ferroelectric capacitor device, preferably, the proportion of the first layer in the plurality of perovskite-like layers is greater than 0 and smaller than 0.3, and 0 less than x less than 0.3 in the general formula (7).
By the above setting, generation of a precipitation can be suppressed substantially completely. This ensures the prevention of the ferroelectric capacitor device from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
The fifth ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers include at least one first layer represented by general formula (8): BO3 (where B is a tetravalent metal) and at least one second layer represented by general formula (9): (A1xe2x88x92xBix)2B3O10 (where A is a trivalent metal, B is a tetravalent metal, and 0 less than x less than 1).
According to the fifth ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film. Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
Preferably, in the general formulae (8) and (9) of the fifth ferroelectric capacitor device, A is a lanthanoide such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb or Lu, and B is Ti.
By the above setting, a ferroelectric film excellent in fatigue characteristic can be obtained. This makes it possible to provide a ferroelectric capacitor device excellent in rewrite endurance.
In the fifth ferroelectric capacitor device, preferably, the proportion of the first layer in the plurality of perovskite-like layers is greater than 0 and smaller than 0.3.
By the above setting, generation of a precipitation can be suppressed substantially completely. This ensures the prevention of the ferroelectric capacitor device from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
The sixth ferroelectric capacitor device of the present invention includes a bottom electrode, a capacitor insulating film formed of a ferroelectric film, and a top electrode, wherein the ferroelectric film has a bismuth layer structure including a plurality of bismuth oxide layers and a plurality of perovskite-like layers alternately put on top of each other, the plurality of bismuth oxide layers are formed of Bi2O2, and the plurality of perovskite-like layers include at least one first layer represented by general formula (10): (A1xe2x88x92xBix)B2O7 (where A is a trivalent metal, B is a tetravalent metal, and 0 less than x less than 1) and at least one second layer represented by general formula (11): (A1xe2x88x92xBix)2B3O10 (where A is a trivalent metal, B is a tetravalent metal, and 0 less than x less than 1).
According to the sixth ferroelectric capacitor device, a ferroelectric film having a bismuth layer structure free from generation of a precipitation can be obtained as the capacitor insulating film. Therefore, the ferroelectric capacitor device is prevented from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.
Preferably, in the general formulae (10) and (11) of the sixth ferroelectric capacitor device, A is a lanthanoide such as La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, and B is Ti.
By the above setting, a ferroelectric film excellent in fatigue characteristic can be obtained. This makes it possible to provide a ferroelectric capacitor device excellent in rewrite endurance.
In the sixth ferroelectric capacitor device, preferably, the proportion of the first layer in the plurality of perovskite-like layers is greater than 0 and smaller than 0.3.
By the above setting, generation of a precipitation can be suppressed substantially completely. This ensures the prevention of the ferroelectric capacitor device from occurrence of a failure due to increase in leakage current or decrease in breakdown voltage.